When transmitting a screen data, differential screen data for only a difference from a previous screen data is transmitted in order to reduce a transmission data amount. In Japanese Patent Application Publication (JP-A-Showa 61-252784), for example, a differential image data and a position data of a difference are transmitted to a receiver side. In Japanese Patent Application Publication (JP-A-Heisei 8-223430), a differential image data is compressed, and the compressed image data and a position data of a differential region are transmitted to a receiver side.
According to the above-mentioned techniques, a transmission data amount can be reduced compared with a case where image data of a current screen is entirely transmitted. However, even when only a differential image data is transmitted, it is difficult to transmit the entire of differential image data if a communication band is narrow. In these techniques, such a situation is not considered and measures to such a situation are not taken.
On the other hand, a technique for performing monitoring easily with a little work amount is known in a monitoring system having a plurality of imaging units and a plurality of image displaying units. For example, Japanese Patent Application Publication (JP-A-Heisei 10-290447) discloses a technique in which temporal change of an output image data in data size is determined for each imaging unit, and the image data outputted from the imaging unit with a greater temporal change is displayed in an emphasis compared with the image data outputted from the imaging unit with a smaller temporal change. However, the technique disclosed in Japanese Patent Application Publication (JP-A-Heisei 10-290447) is not a technique for transmitting a differential image data.